Intracellular microelectrodes are used to study the electrophysiology of single fibers from mammalian hearts and to measure intracellular ionic activities in an attempt to determine the mechanisms responsible for disturbances of rhythm and conduction and the mode of action of cardiac drugs. Emphasis is being placed on studies on depressed fibers, on correlations between intracellular and extracellular activities of Na ion & K ion, transmembrane potentials and drug action. Drugs are applied both extra and intracellularly to identify sites of action.